The logging of geological formations is, as is well known, economically an extremely important activity. The invention is of benefit in logging activities potentially in all kinds of mining and especially in the logging of reserves of oil and gas.
Virtually all commodities used by mankind are either farmed on the one hand or are mined or otherwise extracted from the ground on the other, with the extraction of materials from the ground providing by far the greater proportion of the goods used by humans.
It is extremely important for an entity wishing to extract materials from beneath the ground to have as good an understanding as possible of the lithology of a region from which extraction is to take place.
This is desirable partly so that an assessment can be made of the quantity and quality, and hence the value, of the materials in question; and also because it is important to know whether the extraction of such materials is likely to be problematic.
In consequence, a wide variety of logging methods has been developed over the years. The logging techniques result in measurement of physical and chemical properties of a formation usually through the use of a logging tool or sonde that is lowered into a borehole (that typically is, but need not be, a wellbore) formed in the formation by drilling.
Typically, the tool sends energy into the formation and detects the energy returned to it that has been altered in some way by the formation. The nature of any such alteration is processed into electrical signals that are then used to generate logs (i.e. graphical or tabular representations containing much data about the formation in question).
The logging tools usually are elongate, rigid cylinders that might be 2 m or more in length and between about 57 mm (2¼inches) and 203 mm (8 inches) in diameter.
Different parts of a logging tool perform different functions, with for example one part designed to emit energy into a borehole; and another part intended to detect returned energy in accordance with the broadly stated principle of logging outlined above. In particular when considering those parts of a logging tool that detect and (in many cases) process the returned energy it is often strongly desirable for them to be pressed into intimate contact with the borehole surface (wall). In particular, it is often required for a specific part of the tool to be in such contact.
The same requirement also can arise in relation to the parts of a logging tool that emit energy into the formation; and even if this is not needed, it is often important to know the distance by which the energy-emitting and detecting part(s) of a logging tool are spaced from the borehole surface. The latter is referred to as the tool standoff, in respect of which a correction is frequently applied to the logged data so as to remove from the final log information deriving from the fluid in the borehole as opposed to the surrounding rock. This is because the borehole fluid information may, depending on the nature of the fluid, the type of tool, and the amount of standoff, be a significant perturbation to the data of interest to the log analyst or logging engineer.
Many logging tool designs include devices aimed at “eccentering” and stabilizing them inside the boreholes. “Eccentering” is a term understood by those of skill in the downhole device art, and typically refers to the act of making e.g. a downhole tool non-concentric with the borehole in which it resides.
Single spring stabilizer devices are frequently used. One further type of stabilizer is known as a dual V-bowspring, that forms a part of a density logging tool in which the emitting and receiving parts of the logging tool must be presented in contact with and parallel to the borehole wall.
The dual V-bowspring amounts to a pair of resiliently deformable arcuate members that resemble leaf springs. These members are slidably fixed at either end to the exterior of the logging tool such that they curve convexly outwardly away from the tool between the attachment points.
The ends are attached adjacent one another to the logging tool exterior surface. The arcuate members however diverge from one another in the regions between the attached ends so as to resemble a V-shape when the tool is viewed in cross-section. As a result, two spaced spring members lie between the logging tool and the wall of a borehole in which the logging tool is inserted. The resilient deformability of the members acting between the borehole wall and the logging tool proper provides a force that tends to press the opposite side of the logging tool into engagement or with close proximity to the wall.
The dual V-bowspring while effective in many situations suffers the disadvantage that it increases the tool diameter. This means that a tool including the dual V-bowspring cannot be used to log a well that is of restricted diameter; nor can it be “shuttled” in the well.
In the shuttling technique, a relatively small-diameter logging tool is conveyed over most of the depth of the well inside a shuttle cylinder that moves under fluid pressure inside drillpipe. The tool is caused to protrude from the downhole end of the cylinder once it reaches a location at which logging activity is to commence. Shuttling is particularly desirable since it protects the tool against damage over the major part of its travel from the surface location into the borehole. As a result, the logging tool may be deployed to its operative location very rapidly, thereby reducing the overall time to log a well. Additionally this technique is valuable in highly deviated or horizontal wells where the logging tools cannot descend into the well by gravity alone.
The deformable elements forming part of the V-bowspring however prevent the logging tool from fitting into the shuttle cylinder, the outer diameter of which is limited by the dimensions of the borehole or more commonly drillpipe inserted in it.
Even when shuttling is not required, however the dual V-bowspring can give rise to a particular type of error when for example the well is deviated or it extends essentially horizontally.
In this regard the wellbores that penetrate oil and gas fields for the purpose of exploration and hydrocarbon extraction frequently do not extend directly downwardly into the surface of the Earth over their whole lengths, or even at all in some cases. On the contrary, it is often a requirement for the wellbore initially to extend horizontally or at a slope in order to enter a region of rock containing hydrocarbons before changing direction in order to maximize the length of the wellbore lying within the oil/gas field. Indeed, for this reason and also to avoid undesirable rock types that may be unstable or difficult to drill easily, there may be multiple changes of direction along the length of a wellbore. Moreover, some wellbores may be drilled into mountainsides and may have as a result almost no vertically descending sections.
When a logging tool including a dual V-bowspring is conveyed along an inclined or horizontal section of a borehole there is a risk of the tool being pressed incorrectly against the borehole wall.
For example, when the borehole is horizontal and the tool rotates during deployment the V-bowspring members could engage the lowermost part of the wall such that the logging tool is pressed upwardly so as to engage the upper part of the borehole wall. If as is likely to be the case the requirement is to log the rock that lies beneath the borehole such a situation could be highly undesirable since it would not be apparent until after the logging pass was complete (or largely complete) that the tool was operating “upside down”. Additionally in this situation, the weight of the logging tool acts against the springs, so diminishing the force with which the tool is pressed against the side of the well-bore. The logging operation in such a case would essentially be wasted since the accuracy of the resulting log would be unacceptable. Since logging is an expensive activity (especially if, as is very frequently the case, it is necessary to interrupt rig time in order to obtain a log) any action of the V-bowspring stabilizer that results in inversion of the logging tool, and hence wasted logging time, is highly undesirable.
It is also known to employ a single, rigid, retractable arm as a logging tool stabilizer. Such an arm may be contained in a recess formed in the outer casing of the tool. At one end, the arm is pivotably secured to the logging tool. A mechanism (such as a gear mechanism) is provided for the purpose of forcibly rotating the arm so that it protrudes at an angle to the tool body. In this condition, the resulting, free end of the rigid arm can press into the borehole wall and thereby stabilize the tool.
In use of such a single-arm arrangement, the tool is deployed into the borehole with the arm initially retained in the recess so that no part of the arm protrudes outwardly of the tool. Only once the tool is deployed to the desired location in the borehole is the arm caused to extend and stabilize the tool before logging commences.
Clearly, a tool including such an arrangement may be shuttled in the manner outlined above more readily than a tool including a dual V-bowspring, but the single arm stabilizer nonetheless suffers from disadvantages as explained below with reference to FIGS. 1 to 5.
FIGS. 1 to 4 are schematic, cross-sectional views of a logging tool 10 having a single extensible arm 13 as described. The logging tool 10 is in a section of borehole 11 that extends horizontally.
Logging tool 10 includes at least one energy emitter or receiver, represented by the feature 14, that for accurate use of the tool 10 must be correctly positioned relative to the wall 12 of the borehole 11. The tool 10 illustrated in FIGS. 1 to 4 may for example be a density tool in which the Gamma detector lies inside the tool 10 adjacent a Gamma-transparent window in the wall of the tool having a wear-resistant surface 14. It is generally believed that the surface 14 and indeed a further window via which Gamma radiation is emitted into the formation must be pressed into intimate contact with the wall 12 of the borehole 11 adjacent the rock being logged.
Such a situation is visible in FIG. 1, in which the tool 10 lies at the bottom of the horizontal borehole 11 with the rigid arm 13 extending directly vertically upwardly from the tool to engage the wall 12 at the topmost part of the borehole 11. In this configuration the tool is vertically below the arm and the arrangement is generally regarded as an idealized “good” operational arrangement as signified by the legend in the figure.
However, as shown in FIG. 2, in which the arm 13 is inclined relative to the vertical, it is not necessary for the centre line of the tool/arm combination to extend vertically downwardly. On the contrary as signified by FIG. 2 as long as the Gamma detector window/wear surface 14 is pressed into engagement with the borehole wall in the vicinity of the rock formation to be logged it remains possible to complete a good quality log even though the tool/arm combination is not extending vertically downwardly.
Equally, as shown in FIG. 3, it is possible for the arm 13 to align vertically in the borehole 11 in a way that prevents good quality logging. In this example by reason of the tool/arm combination spanning a chord of the circular cross-section of the borehole 11, and not its diameter, there exists a gap 16 between part of the Gamma detector window 14 and the wall 12. This causes inaccuracies in the resulting density logs even if the orientation of the tool 10 seemingly is correct (by reason of being vertical as shown).
FIG. 4 shows a further sub-optimal mode of operation, in which the tool/arm combination spans a chord of the borehole cross-section and is inclined at an angle. Again this results in the gap 16 that prevents accurate logging by reason of creating an unpredictable degree of standoff and/or causing perturbation of emitted and returned radiation quanta by reason e.g. of interactions with the borehole fluid.
In view of the foregoing, there is a need to improve logging tool stabilizing arrangements in order to prevent or reduce faulty situations of the kinds explained.
According to the invention in a first aspect there is provided a logging tool or toolstring, or an element therefor, comprising a stabilizer including a pair of moveable, rigid, main stabilizer arms that each is pivotably secured adjacent to one another at one end to the logging tool, toolstring or element so as to be moveable between a relatively retracted position on the one hand and a relatively advanced position on the other in which the main stabilizer arms diverge from one another and protrude from the logging tool, toolstring or element so as to present a pair of main arm free ends that are spaced from an outer surface of the tool, toolstring or element and are engageable with a borehole surface, the logging tool, toolstring or element including a mechanism for effecting coordinated, powered, linked movement of the main stabilizer arms between the retracted and advanced positions.
In particular, and importantly, the coordinated movement of the arms preferably is synchronised movement.
When a stabilizer arrangement as defined is used together with a single arm device as previously described, and when the dual arm device is actuated after the single arm one, it tends to prevent the problem of inverted stabilizing of a logging tool.
Conveniently, the mechanism links the main stabilizer arms such that a stabilizing force is applied via the respective arms in proportion to forces exerted by the said arms.
An advantage of this arrangement is that for example when only one of the main stabiliser arms is in contact with a borehole wall 100% (or nearly 100%) of the stabilizing (restoring) force provided by the apparatus is applied via the arm in contact.
In other words, the mechanism is capable of automatically allocating the stabilizing force via the main stabiliser arms in proportion to the borehole wall force experienced by each of the arms.
One example of a mechanical arrangement for achieving this effect is described herein. Numerous further arrangements are possible within the scope of the invention.
Preferably, the tool or toolstring includes formed in an outer surface thereof one or more recesses each for receiving therein at least one of the main stabiliser arms so that in the relatively retracted position the main stabiliser arms are flush with or are recessed relative to the said outer surface.
The apparatus of the invention therefore avoids a drawback of the dual V-bowstring that the diameter of the resulting tool and stabilizer combination is too large to permit shuttling operation. This is because when the stabilizer arms occupy their retracted positions the diameter of the tool is determined by the dimensions of the tool casing, and no part of the stabilizer mechanism at such a time protrudes externally of the tool body. Therefore the tool/toolstring/element of the invention may fit inside a shuttle cylinder without problems.
A further drawback of the dual V-bowspring is that the forces applied by the two springs are independent of each other, and it is highly advantageous that the two arms in the apparatus of the invention to be linked and coordinated as described herein.
The tool/toolstring/element preferably includes formed in the outer surface thereof a pair of recesses in the form of slots that are each capable of receiving a respective rigid arm. This arrangement assures the reliable accommodation of the rigid arms when occupying their relatively retracted positions while permitting their forcible deployment to the advanced positions.
Conveniently, each main arm free end includes a portion that is shaped to promote stable engagement with a borehole surface. Specifically the said portion preferably includes a tip that is flattened in a region that is engageable with a borehole wall.
It is desirable to optimise the angle subtended between the rigid arms of the stabilizer. The following explanation addresses this aspect.
The forces generated by a single arm 13 as in the FIG. 4 situation are modeled in FIG. 5.
FIG. 5 is an analysis of forces diagram in which the FIG. 4 situation is simplified to show the tool/arm combination as a single, straight arm 13 spanning a chord as in FIG. 4. The contact point S of the tool with the wall 12 therefore is positioned at the bottom dead centre of the borehole cross-section, which by reason of ignoring the diameter of the cylindrical tool body may not be strictly accurate but is believed to be sufficient for modeling purposes.
Considering the reaction forces on the arm tip 13a with a sidewall force F, and taking moments about contact point S, the result is a clockwise moment from the normal reaction force from the borehole wall. This moment is resisted by an anti-clockwise frictional resistance moment tangential to the borehole wall.
Thus the restoring moment that tends to rotate the tool back to a position spanning the borehole diameter is given by:F. cos θ.2.r. cos θ. sin θ−F. cos θ.k.2.r. cos θ. cos θ
in which r=well radius and k=coefficient of friction.
Initially assuming that the friction is zero, a plot of this restoring moment against the eccentric angle for different borehole sizes is as shown in FIG. 6 for a unit value of the sidewall force F.
The FIG. 6 plots goes through a peak at about 35 degrees, regardless of the borehole size.
A modified plot is shown in FIG. 7, in which a coefficient of friction between the arm tip 13a and the borehole wall 12 of 0.25 is assumed, being a typical but non-limiting value.
In FIG. 7, there is no net restoring moment until the angle θ, being the angle subtended between the arm 13 and a vertical bisector of the circular borehole cross-section as illustrated in FIG. 5, exceeds approximately 14 degrees. The borehole size-independent maximum force now arises at a value of θ of about 43 degrees.